A standard approach (“gold standard”) in cancer diagnosis is a hystopathologic examination of a tissue sample in question, which is usually done by an experienced hystopathologist in a specialized laboratory. In order to carry out the examination, tissue samples are extracted from the patient by means of a biopsy. A disadvantage of this approach is that it is slow, labor-intensive and causes a considerable discomfort to the patient.
As an alternative to the hystopathologic examination, various infrared spectroscopy methods and in particular infrared Fourier spectroscopy methods for tissue analysis have been proposed. However, the majority of these methods require the extraction of a tissue sample via biopsy, usually under the application of an anesthetic. The extracted tissue sample is subsequently cut in thin slices by a microtom, the slices are further thinned and optionally dried and/or chemically treated and finally subjected to an infrared spectroscopic examination in a transmission or transflection mode. Exemplary infrared spectroscopy methods for tissue analysis are disclosed in the publication Hughes, Caryn [Thesis] Development of Fourier Transform Infrared Spectroscopy for Drug Response Analysis, University of Manchester, 2011.
R. Messerschmidt describes in WO 2009/137122 an Attenuated Total Reflection (“ATR”) prism based spectroscopic method, which utilizes spectral measurements around the critical total reflection angle in infrared range by means of a Fourier transformation spectroscopy. A disadvantage of this method is that it is relatively slow, as it requires a high precision angular scan of the ATR prism and the detection and processing of many spectral images obtained at different angular positions.
Another optical approach for investigating the dielectric properties of materials and in particular of thin films is the ellipsometry. Ellipsometry can be used to measure various material properties, such as chemical composition, electrical conductivity, roughness, thickness (depth), etc. The ellipsometry is based on a detection of the changes in the polarization of reflected or transmitted light as the incident radiation (in a known state) interacts with the measured object. The polarization change may be quantified by various ellipsometric parameters, such as the tangent Psi as the square root of the intensity ratio and the phase difference Delta or a combination thereof.
Spectroscopic ellipsometry (SE) employs ellipsometric measurements of polarization changes (expressed by one or more ellipsometric parameters, such as tangent Psi and Delta), for a number of different wavelengths (or wavelength numbers). The basic principles of the infrared spectroscopic ellipsometry are described in the book of A. Roseler, “Infrared Spectroscopic Ellipsometry,” Akademie-Verlag Berlin, 1990. The author has shown that from the data obtained by infrared spectroscopic ellipsometry, in particular in the mid-infrared range, valuable information about the structure and composition of various objects may be obtained.
The patent publication DE 10 2005 028 894 B3 of A. Roseler and U. Schade discloses a spectroscopic ellipsometer comprising a polarizing interferometer. The patent publication DE 10 2005 062 180 B3 of A. Roseler discloses another spectroscopic ellipsometer comprising a polarizing interferometer in which the polarizer functions as a beam splitter in the polarizing interferometer. Thus, it is possible to improve the accuracy of the measurements, even in cases of unfavorable phase difference. The patent application EP 491 876 Al of E. Garcia-Caurel, B. Drevillon and L. Schwartz discloses an infrared Fourier transformation ellipsometer for identification of biological materials and microorganisms.
An advantage of the ellipsometry is that it does not require a reference, since only the angle between different polarizations is measured. The sensitivity of ellipsometric methods, in particular spectroscopic ellipsometric methods, is very high. One disadvantage of known spectroscopic ellipsometers is that they are rather bulky and less suitable for in-vivo measurements, for example for the purposes of cancer diagnosis.
One object of the present disclosure is to provide an improved, minimally invasive spectroscopic ellipsometer which is suitable for carrying out measurements of biological and technical objects, even at greater depths in the inside of the measured object. In particular, an object of the present disclosure is to provide a minimally invasive spectroscopic ellipsometer for carrying out in-vivo measurements on human or animal tissue, preferably during a surgical intervention. Another object of the present disclosure is to improve the accuracy and reliability of spectroscopic ellipsometry measurements and to reduce the measuring time.
According to an aspect the above objects are solved by the provision of an apparatus for spectroscopic ellipsometry (spectroscopic ellipsometer) and a method for spectroscopic measurements, as defined in the independent claims, respectively. Preferred embodiments are defined in the dependent claims.
Applications of the proposed spectroscopic ellipsometer include but are not limited to in-vivo measurements on human or animal tissue or other biological objects for cancer diagnostic, diagnostic of complex metabolic processes, time resolved reaction kinetic analysis, etc. The proposed spectroscopic ellipsometer is particularly suitable for minimally invasive in-vivo measurements in the depths of the prostate, lungs, stomach, breasts and/or other human or animal organs. The measurements may be carried out during a surgical intervention such as biopsy or surgery, drug administration, tumor screening. Further applications include biological measurements of cells, tissue and/or living organisms for basic biological, medical and/or drug research; forensic, measurements of non-biological materials, etc.
Preferably, the proposed apparatus for spectroscopic ellipsometry enables reliable and traceable measurements at different spatial points of the measured object (including points in the depth of the measured object) with a lateral resolution in the sub-millimeter range. Such spatial resolution is considered to be very good for diagnostic purposes during surgery and sufficient for many other applications. For research applications, in particular under controlled laboratory conditions, the employment of brilliant light sources and diffraction limited optical arrangements, it is possible to obtain a spatial resolution in the upper two-digit micrometer range for the infrared spectral range.
According to an aspect, there is provided an apparatus for spectroscopic ellipsometry (spectroscopic ellipsometer), preferably an apparatus for infrared spectroscopic ellipsometry. The apparatus comprises a light source, a detector, a polarizer, an analyzer, and a measuring probe. The measuring probe comprises an ATR prism (Attenuated Total Reflection prism) having                at least one first surface having at least one measuring portion configured to be brought in optical contact with a measured object, and        at least one second surface having at least one reflective portion.        
The ATR prism is configured so that:                at least a portion of polarized light entering the measuring probe undergoes an attenuated total reflection at the least one measuring portion of the first surface in optical contact with the measured object,        at least a portion of the totally reflected light is reflected back towards the first surface by the at least one reflective portion of the second surface, and        at least a portion of the light reflected back by the at least one reflective portion of the second surface undergoes an attenuated total reflection at the at least one measuring portion of the first surface and is decoupled from the ATR prism,        wherein the difference between the magnitude of the angle (alpha_p) between the first surface and the second surface and the magnitude of the critical total reflection angle is equal or less than 12°.        
The apparatus for spectroscopic ellipsometry employs a novel ATR prism (Attenuated Total Reflection prism) as an optical element for coupling the illumination light into the measured object by means of an attenuated total reflection. The term “ATR prism” is used to cover any optical element that is configured to couple and decouple infrared light into the measured object by means of an attenuated total reflection having a prism, pyramid, prism-plate or other suitable form.
The proposed ATR prism has a surface having a reflective portion thereon configured such that at least a part of the incoming light (i.e. polarized light having passed through the polarizer and optionally through a retarder and entering into the ATR prism through an optical coupling surface of the ATR prism) undergoes twice an attenuated total reflection by/at the measuring portion of the first surface in optical contact with the measured object to form an output (exiting) light, which is subsequently detected by the detector. The provision of a reflective portion for back reflection towards the measuring portion in optical contact with the object, so that the reflected light undergoes an attenuated total reflection for a second time, allows for a considerable enhancement of the measured signal. This improves the sensitivity of the measurements and reduces the measuring time. In the following the first surface, which has a measuring portion in optical contact with the measured object, where measurements are carried out by means of attenuated total reflection will be referred to as a measuring surface. The second surface, which has a reflective portion to guide the light back to the measuring portion, will be referred to as a reflective surface.
In case the ATR prism is monolithically built (i.e. built as a single optical element with fixed surfaces and angles), the provision of a reflective portion has the additional technical effect of the invariance of the average of the magnitudes of the two incident angles alpha_e1 and alpha_e2. The angle alpha_e1 is the angle of incidence of the incoming light at the measuring portion of the measuring surface, i.e. the angle between the normal to the measuring portion and the main ray of the beam of light incident on the measuring portion prior to reflection by the reflective portion of the reflective surface. The angle alpha_e2 is the angle of incidence of the light beam reflected by the reflective portion of the reflective surface at the measuring portion of the measuring surface, i.e. the angle between the normal to the measuring portion and the main ray of the beam of light reflected by the reflective portion of the reflective surface and incident on the measuring portion of the measuring surface.
In other words, the average value alpha_av, alpha_av=(alpha_e1+alpha_e2)/2, is invariant with respect to changes of each individual incident angle alpha_e1+alpha_e2.
In particular, in case of a monolithic prism with an angle alpha_p between the measuring surface and the reflective surface and more precisely between the measuring portion of the measuring surface and the reflective portion of the reflective surface, it holds alpha_p=alpha_av. Accordingly, the average value alpha_av is independent of each individual angle of incidence alpha_e1 or alpha_e2. Thus, the ATR-prism with a reflective surface provided with a reflective portion exhibits invariance of the average value alpha_av with respect to the angles of incidence alpha_e1+alpha_e2. In the following the angle alpha_p will be referred to as prism angle.
Generally, the average value alpha_av may be greater or smaller than the critical total reflection angle, wherein the difference may be equal to or lower than 12°, equal to or lower than 10°, further preferably equal to or lower than 3°. The critical total reflection angle depends on refractive index n_m of the object or medium which is to be measured and the refractive index n_p of the material forming the ATR prism (critical total reflection angle=arcsin (n_m/n_p)). The critical angle may be computed for each particular application of the spectroscopic ellipsometer. For infrared, the refractive index of the material of the ATR prism is typically in the range of 2.38 to 4.1 at the respective wavelength.
As the incident angle approaches the critical total reflection angle, the sensitivity of the spectroscopic ellipsometric measurement increases significantly. For example, the changes of the phase difference Delta as a function of the wavelength (or wavenumber) increase significantly. At the same time, however, the requirements with respect to the precision and stability of the optical arrangement increase and the angular tolerances decrease significantly. Accordingly, the angular orientation of the incident beam at the ATR measuring has to be adjusted and controlled precisely. Further, the requirements with respect to the degree and quality of collimation of the measuring light beam increase and the angular spread of the measuring beam has to be kept very small.
The proposed approach of employing an ATR prism with a reflection portion, configured such that the light undergoes a double attenuated total reflection at a measuring portion increases the robustness of the ellipsometric measurements, since the average value alpha_av of the two incident angles alpha_e1 and alpha_e2 for the first and the second attenuated total reflection is a single value invariant with respect to the changes of each individual incident angle. If for example, the first incident angle alpha_e1 for the first total reflection is reduced due to a tilt of the ATR prism or the axis of the incident light beam (for example after a change of the light source), the second incident angle alpha_e2 is automatically increased. Accordingly, the variation of the first incident angle alpha_e1 may be compensated to a certain extent, although the variation of the first incident angle may cause a considerable nonlinearity in the ellipsometry signals. This allows to guarantee and increase the stability (including the long-term stability) and robustness of the ellipsometric measurement.
Further, the variation of the incident angle alpha_e1 may be easily detected via a detection of the angle beta, which is the angle between the main ray of the light beam incident on the measuring portion of the measuring surface and the main ray of the light after double attenuated total reflection at the measuring portion of the measuring surface. Based on the measurement, a manual or automatic correction of the incident angle may be undertaken to compensate for the variation of the incident angle. The angle beta may range for example from 0° to 24°, preferably from 0° to 5°. Accordingly, the angle beta/2 may range from 0° to 12°, preferably from 0° to 2.5°. Preferably the angle beta is corrected manually or automatically, so that it is set at a small value, preferably a value under 5°, further preferably under 2°. This value enables a good coupling of the incident light beam entering the ATR prism (incoming light beam) and decoupling of the light beam that has undergone a double total reflection.
Preferably, the ATR prism angle alpha_p is adjusted to a specific value depending on the optical properties (e.g. refractive index and absorption coefficient) of expected measured object, for example expected cancer tissue. In other words, the ATR prism may be tuned for a detection of specific objects or materials, such as specific types of cancer tissue. This increases the sensitivity of the detection of these objects or materials.
Generally, the ATR prism may be configured to work at or around the critical total reflection angle. The difference between the critical total reflection angle and the angle of reflection for which the ATR prism is configured may be up to several angular degrees (≈10°), preferably approximately ±3°, more preferably approximately ±1°. Thus for example, reflection angles under the critical total reflection angle (such as for example reflection angles of up to 3° under the critical total reflection angle) may produce valuable measuring data, in particular in combination with a comparison to reference data of the measured object, which are obtained in advance (such as reference data of human tissues having known properties, such as known classification into malign or non-malign). Reflection angles above the critical angle of total reflection may assure that even if there is angular spread of the incoming beam (for example due to dispersion and/or imperfect collimation), the attenuated total reflection is assured for all portions of the incoming light beam, including the outermost rays.
In an example, a monolithic ATR prism, in particular a diamond ATR prism, can be manufactured with a high accuracy, so that the prism angle alpha_p reaches a predetermined value with a tolerance of less than 1/10th angular degrees. Thus, since the prism angle alpha_p is fixed for a given ATR prism, the ellipsometric measurements can be carried out under very stable conditions, which increases the reliability and repeatability of the measurement data, even in case of unstable environment, such as for example during a surgery.
The ATR prism may have a prism, a pyramid or a conical form. In some examples, the ATR prism may be made in a plate-like or optical fiber form. The ATR prism may be arranged in an upright (vertical arrangement) or horizontal arrangement. In the upright direction, the angle between the measuring surface and a longitudinal axis of the measuring probe may be equal to or smaller than 60°, preferably equal to or smaller than 45°, more preferably equal to or smaller than 30°. This allows realizing an ATR prism having both optical and cutting function, as it will be explained in more detail below. In some examples, the ATR prism may be arranged in a horizontal arrangement with the measuring surface being parallel or near parallel to the longitudinal axis of the measuring probe.
The measuring probe may have an elongated form with a longitudinal axis that may be parallel or nearly parallel to the direction of insertion of the measuring probe. In an example, the longitudinal axis of the measuring probe may be at an angle to the insertion direction. This may facilitate the insertion of the measuring probe into the measured object. The measuring probe may have a hollow needle form, a catheter form, a hollow fiber form or any other suitable form, which facilitates the insertion of the measuring probe into the measured object (for example a living organ).
Preferably, the ATR prism and the measuring probe exhibit a diffraction limited optical design. For example the ATR prism may be miniaturized and may have cross-sectional dimensions of about 1 mm×1 mm, preferably 0.5 mm×0.5 mm. The lateral extension of the measuring probe (i.e. the cross section of the measuring probe) may be about 2 mm×2 mm, preferably about 1 mm×1 mm, most preferably around 0.2 mm×0.2 mm. This reduces the damages caused to the measured object and enables spatial measurements at relatively high spatial resolution.
The number of reflections at the reflective surface may be more than one. Generally, the number of reflections is uneven (i.e. 2n+1, with n=1, 2, 3, . . . ). In this case, it is possible to assure that the average angle alpha_av exhibits invariance with respect to each individual incident angle, as described above for the case of one reflection.
The remaining components of the apparatus for spectroscopic ellipsometry, such as a light source, analyzer, polarizer, detector, etc. may be arranged in a known manner. Preferably the apparatus for spectroscopic ellipsometry is a Fourier spectroscopic ellipsometer.
The apparatus for spectroscopic ellipsometry may further comprise a suitable data processor for processing the detected spectral ellipsometry data, for example to determine one or more ellipsometric parameters as a function of the wavelength (or wavenumber). Based on the obtained spectroscopic ellipsometric parameters one or more characteristics of the measured object (such as the complex refractive index) may be determined. Further, an unknown object may be identified or classified by for example comparing the obtained characteristics with reference characteristics.
In the apparatus for spectroscopic ellipsometry, the first surface (measuring surface) and the second surface (reflective surface) may intersect along a common line of intersection, thereby forming a cutting blade for cutting through the measured object. Alternatively, the ATR prism may comprise a cutting blade (cutting portion) for cutting through the measured object. In both cases, the cutting blade may form the tip of the measuring probe that is first inserted into the measured object. Preferably, the cutting angle is equal to or lower than 60°, preferably equal to or lower than 45°, further preferably equal to or lower than 30°.
In the first case, the cutting angle of the cutting blade is equal to the prism angle alpha_p, which is selected such that there is a double attenuated total reflection on the measuring surface and which depends on the refractive index of the ATR prism material and the refractive index of the measured object. Depending on the material of the prism, this may prevent the realization of very sharp cutting blades. One advantage of this arrangement may be that at least one cutting surface or a part thereof may serve as a measuring portion, thereby allowing spectroscopic measurements also at object positions close to or in immediate vicinity of the cutting portion. Thus, the zone around the tip or front end of the measuring probe not accessible to measuring light may be reduced or altogether eliminated, thereby allowing optical measurements in the depth of the measured object.
In the latter case, the ATR prism is formed with a cutting blade with a freely selectable cutting angle. The cutting blade may be made of a non-transparent hart material, which is suitable for cutting through the measured object (such as stainless steel). Preferably, the cutting blade is made of an optically transparent material, such as for example diamond, ZnSe, crystalline silicon, crystalline germanium, etc. Further preferably the ATR prism with the cutting blade has a monolithic structure, i.e. the cutting blade is has fixed mechanical (and in case of transparent blade optical) contact with the remaining, optically active part of the ATR prism. The cutting surfaces of the cutting blade may be in flush with the first and the second surfaces of the ATR prism, respectively (i.e. with the measuring and the reflective surface of the ATR prism, respectively).
One advantage of an ATR prism comprising a cutting blade which is not formed by an intersection of the measuring an the reflective surface (i.e. a cutting blade formed as an element different in its construction than the optically active part of the ATR prism) is that the cutting angle of the cutting blade may be more freely set than the prism angle (which depends on the critical total reflection angle). In particular, the cutting angle (i.e. the angle between the cutting surfaces constituting the cutting blade) may be considerably lower than the prism angle (i.e. the angel between the measuring and the reflective surfaces). Thus, the cutting angle may be preferably equal to or less than 30°, more preferably equal to or less than 25°.
This facilitates the cutting through the measured object and reduces the destructions and injuries induced during insertion of the probe in the measured object. However, in this case parts of the measured objects around the cutting blade may not be accessible to the measuring light.
In both of the above described examples, the ATR has a double function: an optical and a cutting function. An advantage of an ATR prism having a double function is that the optical measurement may be performed in-situ, immediately after the cutting of the measured object. In case of soft tissues and other similar materials, as a rule a thin fluid film or layer is formed on the measuring surface of the ATR prism while the ATR prism (with or without additional cutting blade) cuts through the soft tissue. Due to this thin fluid film or layer, there is a good optical coupling between the cut part of the tissue that is to be measured and the measuring surface. Accordingly, it is not necessary to apply additional high pressure, in order to assure good optical contact. This is a considerable advantage over a horizontal ATR approach, which generally requires the application of high pressure in order to ensure good optical contact between the horizontal measuring surface and the measured object, in particular in case the examined soft tissue has been previously cut by a microtome and the drying process of the cut tissue has already started.
The optically active part of the ATR prism and optionally the whole ATR prism may be made of any suitable optical material that has a sufficient transparency in the relevant spectral range (for example infrared). For example the ATR prism may be made of diamond, zinc selenite (ZnSe), crystalline germanium (Ge), crystalline silicon (e.g. produced by a floating zone process), etc.
In an example, the ATR prism (with the cutting blade) is made of diamond. Diamond is optically transparent for a broad spectral range, has a high refractive index, in particularly in infrared range, and exhibits an excellent durability, chemical inertness and biocompatibility. Due to its high hardness, it allows realizing an ATR prism having a double function: an optical and a cutting function for cutting through the measured object. Further, it is possible to manufacture very small ATR prisms with a high precision. This facilitates the manufacturing of very thin, minimally invasive measuring probes.
Crystalline silicon (Si) also exhibits high hardness and good biocompatibility. An additional advantage is that it has a high refractive index (n=3.4 in infrared), thereby enabling the construction of ATR prisms with very sharp cutting blades having cutting angles equal to or less than 30°. Further, due to the high refractive index, a silicon surface behaves almost like a perfect mirror surface when the angle of incidence is greater than about 60° (total reflection without attenuation or non-attenuated total reflection). This effect may be advantageously used to guide a polarized infrared light coupled to the ATR prism to the measuring portion of the measuring surface.
Crystalline germanium (Ge) has also a very high refractive index in infrared (n=4 for mid infrared range). This allows constructing ATR prisms with very sharp cutting blades having cutting angles equal to or less than 25°. Further, as in case of crystalline silicon, it allows to advantageously use the mirror like behavior at high incident angles (for example higher than) 50°, to guide the light coupled to the ATR prism to the measuring portion of the measuring surface.
For example, the first and/or the second surface may comprise at least one reflective portion, configured to reflect incident light by means of non-attenuated total reflection towards the second surface. Thus, it is possible to easily realize an optical guide like structure without the application of a reflective layer, thereby reducing the costs of the ATR prism and simplifying the production.
Of course, the first and/or the second surface may comprise at least one reflective region provided with a reflective layer. The reflective layer may be for example a metal layer, such as an aluminum layer. The reflective layer may have a multilayer structure, preferably with a hard coating layer as an outermost layer.
In an example, the first surface may comprise a plurality of measuring portions, each of them configured to be brought into optical contact with the measured object. This improves the detected measurement signal and enables reducing the measurement time. However, the lateral spatial resolution may be reduced, since the measurement signal is collected from different spatially distant sites.
In an example, the ATR prism may comprise a plurality of surfaces, each having at least one measuring portion configured to be brought in optical contact with a measured object. The apparatus for spectroscopic ellipsometry may also comprise a plurality of surfaces, each having at least one reflective portion. In an example, the reflective portions may be provided on each of the surfaces comprising the at least one measuring portion.
For example, the ATR prism may exhibit a pyramid form and the pyramid faces may have at least one measuring portion and at least one reflective portion, arranged such that there an attenuated total reflection occurs twice at each measuring portion provided on each of the pyramid faces. Thus, it is possible to realize a very compact multi-channel ATR prism that is capable of simultaneously measuring the spectroscopic polarization properties at at least two different sites of the measured object. At the same time the pyramid faces meeting at the apex of the pyramid may serve as cutting surfaces for cutting through the measured object.
In some examples, the number of reflections at each reflective surface may be one. The number of reflections at the reflective surfaces may be more than one, for example three, five, etc.
The spectroscopic ellipsometric measurement may be combined with further optical measurements, all of them using advantageously the same ATR prism as an optical coupling element. To this end the ATR prism may comprise a plurality of optical coupling surfaces. The ATR prism may comprise a first optical coupling surface for coupling light (polarized light) for the spectroscopic ellipsometric measurement into the ATR prism and optionally decoupling the light after an interaction with the object out of the ATR prism. The ATR prism may further comprise one additional (second) optical coupling surface for coupling light for further optical measurements into the ATR prism (and optionally decoupling the light for further optical measurements after interaction with the measured object out of the ATR prism). The further optical measurement may include but is not limited to any of Raman spectroscopy, optical coherence tomography, swept-source spectral-domain optical coherence tomography, microscopic observation (preferably in ultraviolet, visible or near infrared light).
For example, the spectroscopic ellipsometric measurement may be combined with Raman spectroscopy measurement. Preferably, the ATR prism is configured such that the Raman excitation light emitted by a suitable light source (for example light source emitting light in the near infrared (NIR), visible (VIS) or ultraviolet (UV) range) passes through the second coupling surface and is directed to the measuring surface of the measuring probe. The scattered Raman light may exit the ATR prism through the second coupling surface or through a respective exit surface for the Raman scattered light and be directed to a suitable detection path for the Raman spectroscopy.
Similarly, it is possible to combine the spectroscopic ellipsometric measurement with other measurements, such as optical coherence tomography (in particular swept-source spectral-domain optical coherence tomography), microscopic observation (preferably in visible or near infrared light) or any other optical measurements. The ATR prism may comprise more than two optical coupling surfaces for the different types of optical measurements.
The optical coupling surfaces may be plane or curved surfaces (for example convex or concave curved spherical or aspherical surface). Preferably, the measuring probe and the ATR prism are configured such that the light for further optical measurement is incident normally or near normally on the measuring portion of the measuring surface. For example, the at least one optical coupling surface for coupling light for further optical measurement may be arranged such that this surface or a tangential plane to this surface is normal or nearly normal with respect to the measuring surface. Nearly normal incidence means incidence at an angle deviating from the normal (perpendicular) incidence by no more than ±15°, preferably by no more than ±12°. Due to the normal or near normal incidence of the light for further optical measurement to the measuring portion of the measuring surface, this light does not experience a total reflection, so that the photons returned from the measured object may pass through the measuring surface and be directed towards a suitable detector (i.e. directed in a suitable detection path for second optical measurement).
The apparatus for spectroscopic ellipsometry may comprise additional light sources (such as additional light sources for Raman excitation light, optical coherence tomography light and/or microscopic observation light), detectors and additional optical forming additional illumination and detection paths. Thus, it is possible to combine at least two different optical sensors in the same optical arrangement and to carry out further optical measurements, preferably simultaneously with the spectroscopic ellipsometric measurements. The detected data of the different optical measurements may be combined and used to determine the properties of the measured object. This may for example improve considerably the identification of unknown objects, for example the classification of an observed tissue as healthy tissue, cancer tissue, etc.
The apparatus for spectroscopic ellipsometry may comprise a kit or a magazine of measuring probes, wherein preferably each of the probes exhibits an ATR prism with different prism angle (i.e. different angle between the measuring and the reflective surface). The different probes may be easily exchanged when the need arises. In an embodiment, due to the invariance of the average value alpha_av, the alignment of the optical system after a change of the measuring probe is relatively simple and may be carried out automatically.
The apparatus for spectroscopic ellipsometry may employ different light sources, such as broadband light sources emitting light with a substantially continuous spectrum or broadband light sources emitting monochromatic or quasi-monochromatic light with a variable (i.e. scannable) wavelength within a broad spectral range (e.g. so called swept sources). The emitted monochromatic or quasi-monochromatic light may be frequency and/or amplitude modulated. The broadband light source may be combined with a monochromator, a spectrometer or an interferometer, as in known in the art.
The broadband light source may be for example a brilliant light source, such as a synchrotron radiator (emitting preferably in mid-infrared), a broadband quantum cascade laser or a laser battery. Through the use of such brilliant light sources, it is possible to achieve a diffraction limited optical design and miniaturize the measuring probe. Further, the overall measuring time may be reduced. This facilitates the application of the spectroscopic ellipsometer for in-vivo tissue analysis, for example during a surgical intervention.
If quantum cascade lasers or other laser sources are used, it is preferable to reduce the time coherence of the illumination light. This reduces or prevents the occurrence of parasitic interferences causing errors in the ATR spectroscopic measurements.
The spectrometer may be any conventional spectrometer, for example a conventional infrared Fourier transform spectrometer. An exemplary spectrometer is disclosed for example in DE 10 2014 002 514. The interferometer may be for example a Michelson-interferometer, a rotatable-mirrors based interferometer or any other suitable interferometer. Preferably, the apparatus for spectroscopic ellipsometry is a Fourier spectroscopic ellipsometer.
The detector may be an integral detector, for example a spatially and/or spectrally integrally detecting detector. The detector may be a one or two-dimensional detector array with a plurality of detector elements. Suitable detectors for light in the infrared spectral range are for example mercury-cadmium telluride (MCT) detectors. Suitable detectors for light in the visible range are for example CCD cameras.
Further, the apparatus for spectroscopic ellipsometry may comprise a data analysis component configured to process the obtained spectroscopic ellipsometer data to obtain one or more ellipsometric parameters as a function of the wavelength (or wavenumber) of the illumination light. The ellipsometric parameters may include tangent Psi and Delta (tangent Psi is the square root of the intensity ratio and Delta is the phase difference) or a combination thereof, each as a function of the wavelength (or wavenumber) of the illumination light. The analysis may include a Fourier transform spectrometric analysis or any other suitable analysis. Further, the analysis may comprise a Principal Component Analysis or other suitable statistical analysis to obtain characteristic features, which may be then used to identify or classify an unknown measured object. Research carried out by the inventors on tissue samples has shown that based on the obtained ellipsometric data (in particular infrared ellipsometric data) significant information concerning the bio-molecular composition of the tissue sample may be obtained.
According to a further aspect, there is provided a method for spectroscopic ellipsometric measurement by using the apparatus for spectroscopic ellipsometry according to an aspect of the present disclosure. The method comprises:                bringing the at least one measuring portion of the first surface in optical contact with the measured object;        illuminating the at least one measuring portion with incident light, so that at least a portion of the incident light undergoes an attenuated total reflection by the at least one measuring portion,        reflecting, by the at least one reflective portion of the second surface, at least a portion of the totally reflected light back towards the measuring portion, whereby at least a portion of the light reflected back by the reflective portion undergoes an attenuated total reflection by the at least one measuring portion of the first surface;        decoupling the totally reflected light from the ATR prism,        detecting at least a portion of the light exiting the ATR prism; and        determining at least one ellipsometric parameter as a function of the wavelength of the incident light.        
As explained above, after having passed through a polarizer and optionally a retarder the polarized incoming light is coupled to the ATR prism, where it undergoes twice an attenuated total reflection on the boundary between the measuring portion of the measuring surface of the ATR prism and the measured object. The light exiting the ATR prism may pass through an analyzer and optionally a retarder and/or further optical elements (such as filters, lenses, etc.) prior to being detected and subjected to further analysis to obtain ellipsometric parameters (e.g. Delta, tangent Psi or a combination thereof) as a function of the wavelength (or wavenumber) of the illumination light, as known in the art. The ellipsometric measurements may be carried out for a plurality of angular positions (preferably at 0°, 45°, 90° and 135°) of the analyzer with respect to the polarizer.
The analysis of the obtained spectroscopic ellipsometric data may include a Fourier transform spectrometric analysis or any other suitable analysis. Research carried out by the inventors on tissue samples has shown that based on the obtained ellipsometric data (in particular infrared ellipsometric data) significant information concerning the bio-molecular composition of the tissue sample may be obtained.
The ellipsometric measurements may be repeated for a plurality of different measurement sites or regions in and/or on the measured object. Thus, the method for spectroscopic ellipsometry may include moving the measuring probe to a new region or new measurement site of the measured object and repeating the steps of bringing the at least one measuring portion of the measuring surface in optical contact with the measured object, illuminating the at least one measuring portion, reflecting, by the at least one reflective portion of the reflective surface, at least a portion of the totally reflected light back towards the measuring portion of the measuring surface, where it undergoes a second attenuated total reflection, decoupling the totally reflected light from the ATR prism and detecting at least a portion of the exiting light.
By repositioning the probe (i.e. spatially translating the probe), it is possible to conduct a multiple measurement at a plurality of spatially separated points within and/or on the measured object, so as to obtain a spatial scan of the ellipsometric parameters as a function of the wavelength (or wavenumber). The distance between the measuring points (i.e. the density of the spatial measurements) as well as their arrangement may be freely selected according to the specific application. Since the measuring probe and more specifically the ATR prism may be miniaturized, the spatial resolution of the scan may be very relatively high, for example in the sub-millimeter range down to two-digit micrometer range.
Based on the obtained data, it is possible to determine the two- or three dimensional spatial distribution of the refractive index “n” and the absorption coefficient “k” (i.e. the imaginary part of the complex refractive index) of many different objects, such as for example living organs. The obtained information is much richer than the information obtained solely on the basis of spectroscopic measurements (for example spectroscopic absorption measurements).
The spectroscopic ellipsometric measurement may be combined with further optical measurements, all of them using advantageously the same ATR prism.
For example, the spectroscopic ellipsometric measurement may be combined with Raman spectroscopy measurement. The method may comprise illuminating at least one portion of the measuring surface of the ATR prism in contact with the measured object with Raman excitation light, wherein the Raman excitation light is incident on the illuminated portion perpendicularly (normally) or at an angle deviating from the perpendicular (normal incidence) by no more than ±15°, preferably by no more than ±12°; detecting at least a portion of the Raman scattered light and subjecting the detected light to a Raman spectroscopy analysis.
The Raman excitation light may be emitted by a suitable light source (for example a light source emitting light in the near infrared (NI), visible (VIS) and ultraviolet (UV) range) and be coupled into a Raman spectroscopy illumination path including the ATR prism. Due to the normal or near normal incidence of the Raman excitation light to the measuring portion for Raman spectroscopy of the measuring surface, the incident Raman excitation light does not experience a total reflection, so that the excited Raman photons may pass through the measuring surface and be directed towards a suitable detector for scattered Raman light (i.e. directed in a suitable detection path for the Raman spectroscopy). The Raman spectroscopy illumination and/or detection paths may have further common components with the spectroscopic ellipsometry illumination and detection path (other than the ATR prism).
It is also possible to combine the spectroscopic ellipsometric measurement with optical coherence tomography measurements, in particular swept-source spectral-domain optical coherence tomography. The method may comprise illuminating at least one portion of the measuring surface of the ATR prism in contact with the measured object with optical coherence tomography illumination light, wherein the optical coherence tomography illumination light is incident on the illuminated portion perpendicularly (normally) or at an angle deviating from the perpendicular (normal) incidence by no more than ±15°, preferably by no more than ±12′; detecting at least a portion of the light returned back from the measured object and subjecting the detected light to an optical coherence tomography analysis.
Due to the normal or near normal incidence of the optical coherence tomography illumination light (for example an illumination light in the visible (VIS) or near infrared (NI) spectral range) to the measuring surface, the incident optical coherence tomography illumination light does not experience a total reflection, so that the light returned back from the measured object may pass the measuring surface and be directed in the direction of a suitable detector for the optical coherence tomography light (i.e. directed in a suitable detection path for the optical coherence tomography).
Still further it is possible to combine the spectroscopic ellipsometry measurements with microscopic observation of the measured object, for example in the visible spectral range, in the near infrared spectral range or in the ultraviolet spectral range. In this case the ATR prism and the measuring surface of the ATR prism may be used in the illumination and detection path of the microscopic system. This allows observations of the measured objects on a microscopic scale, while carrying out spectroscopic ellipsometry measurements. Further it is also possible to carry out image processing on a microscopic scale, preferably in real-time, for example to control the progress of a surgical intervention.
The method may comprise illuminating the at least one portion of the measuring surface of the ATR prism in contact with the measured object with illumination light in the visible spectral range, wherein the illumination light is incident on the illuminated portion perpendicularly (normally) or at an angle deviating from the perpendicular (normal) incidence by no more than ±15°, preferably by no more than ±12°, detecting at least a portion of the light returned back by the measured object, thereby forming a microscopic image of the measured object.
A combination with other optical sensors and optical measurement methods is also possible.
In the above examples, it is possible to obtain data by a plurality of different optical measuring method from a substantially the same measuring site of the object. In this case the illuminated portion of the measuring surface for further optical measurement may overlap at least partially with the measuring portion for infrared spectroscopic measurement by double attenuated total reflection. However, it is also possible to provide an ATR prism, in which the different optical measuring methods use different portions of the measuring surface. For example, the measuring portion for the attenuated total reflection infrared spectroscopic measurement may not coincide with the area used for the additional optical measurements. Further, in the above examples, it is not necessary that the light used for the additional optical measurements uses the whole area of the measuring portion used by the attenuated total reflection infrared spectroscopic measurement.
The above and other objects, features and advantages of the present disclosure will become more apparent upon reading of the following detailed description of preferred embodiments and accompanying drawings. Other features and advantages of the subject-matter described herein will be apparent from the description and the drawings and from the claims. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments.